Powder metallurgical method of shaping articles from high melting metals



c. G. GOETZEL 2,714,556 POWDER METALLURGICAL METHOD OF SHAPING ARTICLES FROM HIGH MELTI-NG METALS Aug. 2, 1955 2 Sheets-Sheet 2 Filed Nov. 25, 1950 POWDER METALLURGICAL METHOD OF SHAP- IN G ARTICLES FROM IHGH MELTING METALS Claus G. Goetzel, Yonkers, N. Y., assignor to Sintercast Corporation of America, Yonkers, N. Y., a corporation of New York Application November 25, 1950, Serial No. 197,605 11 Claims. (Cl. 75-203) This invention relates to the manufacture of articles shaped from high-melting metals, metal alloys and metal compounds. It is a continuation in part of copending patent application Serial No. 787,514, filed November 22, 1947, in the joint names of John L. Ellis and Claus G. Goetzel, now abandoned.

In the manufacture of heat resistant metallic articles and parts thereof, such as blades, buckets, vanes for jet engines, rockets or gas turbines, powder metallurgical methods are being extensively used in order to meet the high temperature and mechanical requirements.

The customary processes of this type employing simple pressing and sintering operations of the high melting metal powders have their limitations due to the inherent porosity of the articles, their fine grain size and their weak grain boundaries, all of which contribute to the creation of unfavorable tensile strength, fatigue strength, and creep resistance conditions at elevated temperatures.

Another difficulty involved in the customary metal powder compacting methods consists therein, that with shapes having tapering cross sections terminating in feather-like edges the pressing operation requires the exertion of force at right angles to the feather edge. Under such circumstances, the sharp edges of the punches are liable to hit each other inside of the die cavity and thus be damaged. It is true, that such feather edges may be produced by exerting force in the direction of the feather edge; however, in this case other difficulties arise because of the excessive friction encountered in ejecting the compact from the die cavity due to the wedging action of the powder at the high operating pressure.

In order to remedy these difficulties, the use of powders of greater plasticity and improved flow characteristics has been suggested; but these measures have limitations, because powder flow would occur only if the driving force is constituted by a considerable pressure difference in the formation of the compressed powder bodies; this will result in undesirable density variations in the finished compacts.

One of the objects of this invention is to provide a new method of forming articles, especially those having intricate shapes, by powder metallurgy whereby improved physical characteristics of the finished product can be obtained.

Another object of the invention is to provide a powder metallurgy method particularly adaptable for producing articles of high heat resistant properties, including such properties as high hot tensile strength, high hot fatigue strength and high resistance to creep at elevated temperatures.

The invention includes an operation known as impregnation or infiltration which involves the introduction of a lower melting semi-refractory, corrosion resistant, liquid metallic phase into a skeleton body of a higher melting metal, metal alloy or compound.

In these powder metallurgy techniques involving impregnation or infiltration of a formed skeleton body, the powdered high melting metal, metal alloy, or metal compound is usually shaped into the form of a skeleton and then removed from the forming mold before sintering. After the skeleton has been formed and sintered, a second States Patent 'ice or auxiliary lower melting metal is brought into contact with the skeleton in a suitable ceramic or metallic container and heat applied, so as to liquefy the second metal, and cause the second metal to be drawn into the skeleton by capillary action.

Difficult problems arise, however, in the forming'of the skeletons and this is particularly so when the shape is complex and irregular, due to lack of uniformity in the pressed skeleton because the particles of the powdered material will not be distributed evenly within the mold so that the density of the skeleton will be non-uniform. In prior impregnation processes, sizing and machining frequently have been necessary following the impregnation step because of the uneven distribution of the second metal on the surface of the skeleton due to adherence of the second metal to the surface of the skeleton.

Density variations in the skeleton of the aforemene tioned types would result in a non-uniform strength, nonuniform shrinkage, and deformation in the sintering proc ess. The density variation also would result in localized tool pressure in coining operations, non-uniform elastic spring back, and non-uniformity in hardening, or subsequent heat treatments of the compact or composite body..

One of the features of the invention is to form the skeleton by placing the powdered material, preferably of selected particle size and distribution, in a mold and then to compact the powdered material in the same mold in such manner as to obtain a body having a critical pore volume concentration which will be further explained below.

By pore volume concentration obviously is meant the volumetric proportion of the voids as compared to that of the solid particles. One method of producing such bodies with critical pore volume concentrationhas been found to be the so-called dynamic loading or rampacking method, with or without the application of static pressure. Due to the use of the same mold during the rampacking, sintering and infiltrating period, the originally produced pore distribution and pore volume concentration will remain unchanged. Moreover, manipulation of highly porous and fragile skeleton bodies leading, as will be shown later, to superior properties, is eliminated by the use of the same mold. The further development of this single mold manufacturing principle is another important object of this invention.

The control of the pore volume concentration and of the pore distribution in the sintered skeleton bodies is, as stated above, an essential postulate in the production of articles by the infiltration or impregnation of powder metallurgical bodies which must be resistant to high stresses, and high temperatures, as for instance turbine blades, buckets, nozzles, and vanes.

The physical and mechanical properties of the articles produced by the metal infiltration of rampacked and sintered metal powder bodies do not change in direct relationship with the pore volume concentration. Indeed, an optimum of the desired properties in the final articles, such as particularly a satisfactory stress resistance, is only attained, if a freely interconnected system of uniformly distributed pores is maintained throughout the entire skeleton bodies causing their uniform and thorough infiltration; this pore structure, however, does not coincide with the reduction of the pore volume concentration and a minimum of pore volume concentration in the sintered bodies is not at all desirable.

Optimum properties will only result in articles produced by powder metallurgy methods if a certain type of pore structure is maintained throughout the entire powder and skeleton bodies, respectively. Processes to obtain such pore structure will be described later.

The invention is based on the recognition that this type of pore structure can only be obtained if the compacting or rampacking work is limited to certain well-defined ranges.

These limits vary with the high-melting materials within comparatively small ranges; they will be denoted in the following as the range of critical pore volume concentration.

This critical pore volume concentration, therefore, defines within the realm of this invention a pore concentration of the compacted or rampacked powder bodies where all pores are freely interconnected and evenly distributed throughout the entire powder body; the infiltrating metal will, therefore, form an evenly distributed network and act as a cementing binder.

As the maintenance of the critical pore volume concentration may not warrant sufiicient strength and resistance to impacts of the compacted powder bodies, it is important that the same remain in the same mold during the compacting, sintering and infiltration steps, so that breakage of the compacted powder bodies, or of the skeleons is avoided.

It is a further object of the invention to set out the working limits and the particular processing conditions which control the attainment and the maintenance of the critical pore volume concentration.

It is also an object of the invention to improve, by the maintenance of the critical pore volume concentration, the structure uniformity of machine parts and similar articles produced by powder-metallurgy processes and particularly of those having a complicated shape, such as tapering sections terminating in thin edges.

It is another object of the invention to eliminate, by the establishment of the critical pore concentration, uncertainties with regard to the applicability of the various high melting materials for certain purposes and to secure a production line manufacture of machine parts by powder metallurgical methods.

In order to secure the maintenance of the critical pore volume concentration in the compacted and sintered skeleton bodies, particularly in conjunction with intricately shaped bodies, a hydraulic press may be employed and means connected with said press to impart to the filled mold a pressing and a vibratory or dynamic loading action. The dynamic loading can be in the form of an impact or sudden force application. For the removal of bridging and interlocking particles of the refractory metal powder the hydraulic press may be provided with pulsating pressure devices, so as to impart an impact type of pressing action in conjunction with a jolting action, and thereby to achieve the desired flow to the powder.

After the porous body has thus been formed in the mold, it is then transferred to a suitable sintering furnace.

A pre-sintering heat treatment may be applied to the skeleton bodies which will produce alloying to a certain degree if the skeleton is composed of two substances. The sintered skeleton is now ready for infiltration or impregnation with a lower melting metal or metal alloy, which may be placed in contact with the sintered body; the temperature is then increased to slightly above the melting point of the infiltrant, so as to properly cause the flow thereof into the interconnected pore system of the skeleton. For that purpose a pressure differential apparatus such as pressure applying devices, a vacuum, or a centrifugal device, can be used.

After the infiltration and cooling, the shaped article is removed from the mold.

The invention will now be described more in detail in connection with certain typical powder materials and with reference to the attached drawings, showing a preferred embodiment thereof.

In the drawings,

Fig. 1 is a schematic sectional view of a mold for an article such as a turbine blade;

Fig. 2 is a sectional view taken along the line 22 of Fig. 1;

Fig. 3 is a diagrammatic view of one form of apparatus which may be used for applying pressure and dynamic loading to the mold;

Fig. 4 is a diagrammatic view of a sintering furnace with a mold therein;

Fig. 5 is a schematic section view of an impregnating apparatus wherein fluid pressure is used to assist in the impregnation;

Fig. 6 is a view wherein mechanical pressure is used in impregnating the skeleton;

Fig. 7 is a flow diagram illustrating one manner of carrying out the invention.

The invention is especially adapted for use in producing objects of intricate, or complex shapes; it is by way of example illustrated in conjunction with the manufacture of a turbine blade; however, it is to be understood that any other type of article, such as a turbine bucket, nozzle, vane, valve, etc. may be produced in accordance with the invention.

In the mold shown in Figs. 1 and 2, the cavity 10 for the turbine blade has a feather edge 11, a curved body portion 12 and a base 13 of the desired shape. The side wall 14 of the mold can be formed with a suitable taper so that the mold will be receivable in the mold holder as explained hereafter and so as to be withdrawable therefrom.

The selected powdered material to be used for the production of the skeleton is fed into the mold prior to its being placed in the compacting apparatus.

The particle size is also important in the production of a body having the critical pore volume concentration. An example of a suitable average particle size analysis would be: 9% less than one micron, 80% ranging between one and five microns, 8% between six and ten microns, 3% between eleven and twenty microns, and

0% over twenty microns; another example would be 55% less than one micron, 30% ranging between one and five microns, 8% between six and ten microns, 6% between eleven and twenty microns, and 1% over twenty microns.

Three categories of powdered materials are specifically considered for the production of the skeleton in conformity with the invention.

The first category consists of the high melting metals, such as tungsten, molybdenum, tantalum, columbium, titanium and zirconium.

The second category consists of alloys, which have one of the aforesaid refractory metals as a major constituent. Alloys of this kind include tungsten and molybdenum alloys containing for example 25, 50 and 75 percent, respectively, of molybdenum; tungsten and chromium alloys containing 5 to 35 per cent chromium, molybdenum and chromium alloys containing 5 to 50 per cent chromium.

The third category includes titanium carbide, tungsten carbide, zirconium carbide, molybdenum carbide, tantalum carbide, columbium carbide, as well as combinations, such as solid solutions or multiple compounds of titanium carbide and chromium carbide, tungsten carbide, or zirconium carbide, titanium carbide and tantalum carbide, titanium carbide and columbium carbide, or titanium carbide and tantalum carbide and columbium carbide.

After the initial powders have been fed into the mold, the latter may be placed into a hydraulic press arrangement such as diagrammatically shown in Fig. 3. The mold 15 with powder therein is placed in the mold holder 16, said mold holder 16 being slidably carried in mold holder support 17. The mold holder support 17 is carried in the press bed 18 and held in place by a clamping ring 19 in any suitable manner. The mold holder 16 has a tapered aperture 20 therein of suitable taper to match the taper of the mold. A spring guide 21 can be fastened by means of bolts 22 to the bottom of the mold holder, said spring guide 21 retaining a suitable spring 23 between flange 24 thereof and the bottom of the press bed which may have an inset ring 25 therein. The mold holder 16 has a Space leftbetween the. top .25 thereof and the bottom 27 of the clamping ring 19 so that the mold holder can be given a reciprocating movement in the press bed. A mold holder cap ring 26 can be engageable by screw threading in the top of the mold holder 16 so as to hold the mold in position in the mold holder 16. A suitable spacer ring 28 may be provided between the top of the mold and the cap 26 of a suitable composition to evenly apply the pressure exerted by cap 26 to the fragile and irregular mold 15;

A suitable impact or pressure ram 29 can be operated by the hydraulic mechanism of the press, the end 30 of said ram being enterable into the aperture 31 of the cap 26 and into the cavity of mold 15.

A jolting ram 32 is engageable with the flange 24 of the spring guide 21. Spring 23 normally maintains the mold holder in its lowermost position. The jolting ram then can be operated in a reciprocating fashion so as to cause a reciprocation of the mold holder and mold therein, the reciprocating amplitude being governed by the distance between faces and 27. The rams may be furnished with pressure in any desired and conventional manner so that as the main pressure is applied to the ram 29, a dynamic loading can be applied by the jolting ram 32, for example, by applying a pulsating hydraulic pressure thereto. In this manner, a constant pressure force on the powder in the mold can be exerted in conjunction with a vibratory or dynamic loading and this will serve to properly and uniformly compact the powdered material therein.

Other types and manners of producing pulsating or dynamic action may be used, such as for example, a vibrating or jolting table or platen (not shown), operating with varying amplitude and frequency. The dynamic loading or force in some instances should be of a sudden or impact type of load superimposed on the main pressing action.

It is next necessary to sinter the skeleton and this is accomplished by transporting mold 15 with the compacted skeleton therein to the point or zone of sintering, which may be any type of sintering furnace 33, such as illustrated schematically in Fig. 4. In the form shown, a belt 34 is indicated for transporting the mold 15 through the furnace and the furnace 33 may have a protective atmosphere, such as hydrogen, nitrogen, helium or argon, maintained therein. Various methods and apparatus can be employed to transport or hold the mold with the skeleton therein in the furnace.

In some instances, the mold with the skeleton therein may be subjected to a pre-sintering treatment in a protective atmosphere for the purpose of diflusion alloying. The temperature employed, for example, may be very high, such as 1500 C. to 2000 C. The type of heat treatment and sintering atmosphere will depend upon the particular object being made, and materials used.

Upon completion of the sintering operation, the compacted and sintered skeleton in the mold is ready for impregnation. The impregnating material may be iron, nickel, cobalt, chromium, and their alloys with each other, or their alloys with refractory metals, such as tungsten, molybdenum, tantalum, etc., or compounds of such refractory metals as minor constituents.

The infiltration may be accomplished by'placing the mold used for pressing in a suitable furnace, and in contact with an allotted amount of the infiltrant. The infiltrant is brought to a liquefying temperature and infiltration Will take place because of capillary action in the skeleton.

In many cases it is desirable to create an impregnation by a pressure differential between the infiltrant and the skeleton in order to assist the capillary action of the skeleton. An example of one manner in. which a pressure diiferential can be accomplished is shown in Fig. 5 wherein mold 15 with the compacted and sintered skeleton 36 is placed in a furnace or receptacle 37, cover 38 then is 6 placed thereon, an pre e x r in any desired ay on the infiltrant 39 through pipe 40.

It is evident that various manners of causing a pressure differential to exist in the impregnating step can be employed, such as for example, a direct mechanicalpressure as seen in- Fig. 6 wherein mold 15 with sintered skeleton 36 is placed in a receptacle 41 and heat applied thereto by means, for example, of the high frequency heating coil 41a. The receptacle 41 may have a gas-tight housing 42 with a water cooled cover 43 fastened thereto. The cover has a suitable packing gland 44 through which the pressure plunger 45 can reciprocate. Within the receptacle 41 there can be a tube 46 which may be made of some material, such as quartz for example, and the space between the tube 46 and the outer wall 42 may be filled with a suitable insulating powder. A graphite retainer tube 47 can be employed to hold the mold holder 48 in the desired position Within the housing 41. A graphite cover 49 having an aperture 50 therein through which the plunger operates can be placed on top of the mold holder after the mold 15 is in place. A protective gas inlet can be located at 50A for furnishing protective gas, such as hydrogen, nitrogen, helium or argon, to the housing. The infiltrant is located at 51 on top of the skeleton, and can be brought to a liquefying temperature by the heating coil. The graphite mold holder 48 is heated by the high frequency coil 41a, and the heat from holder 48 is transmitted through mold 15 to the skeleton 36 and to the infiltrant 51. Upon application of pressure to plunger 45, the impregnation of the skeleton will be caused by means of a pressure differential thereto.

The same result also may be obtained by the use of a vacuum on the skeleton, so as to cause impregnation thereof. A still further manner in which the pressure differential can be exerted is by means of a centrifugal casting apparatus, not shown.

Following the infiltration or impregnation, the body then may be removed from the mold, which in the case of the turbine blade illustrated would advantageously be of a split design which would permit an opening thereof without its destruction, unless a physical or chemical destruction of the mold is practical.

A further heat treatment may be desirable for the article depending on the materials used for the skeleton and the materials used for the infiltration or impregnation thereof, to produce a solid stable structure of optimum strength, toughness and resistance to deformation at high temperatures. Such a final heat treatment should be conducted in a protective atmosphere, such as hydrogen, nitrogen, helium or argon. The melting point ofthe infiltrant is fairly high, for example, about 1400 C. and it is believed essential for good performance of the composite structure during actual service at a temperature, for example, of 800 to 1000 C., to increase the melting and boiling point of the composite material. It is desirable to eliminate the liquid phase at infiltration temperature when a complete penetration of the pores of the skeleton has been attained. Such can be achieved only by a diffusion causing the formation of a new alloy or metallic compound between the skeleton and the infiltrant of a higher melting point than the temperature of the liquid phase used for impregnation. Such a mechanism of consolidation exists in a number of alloy systems, such as the consolidation of tin in a copper skeleton, or infiltration of aluminum into a nickel skeleton.

Also, if precipitation hardenable alloys are used as the infiltrants, and the infiltrant phase is of a major proportion, the composite body may be precipitation hardened, the precipitation hardening taking place, for example, in the infiltrant itself.

The flow diagram of Fig. 7 shows schematically a sequence of steps which may be employed in the invention, wherein first there is the pressing and dynamic loading of the mold, then there is a transfer of the compacted skeleton in the same mold to a siutering point where the sintering operation can take place. By keeping the skeleton in the mold and impregnating therein, the size is maintained eliminating the necessity for machining or further sizing operations in many instances. If desired, there may be a pre-sintering operation before the sintering is performed. Following sintering, impregnation of the skeleton is performed while the skeleton is still in the same mold and as mentioned previously, the impregnation may be due to capillary action or may be performed under a pressure differential or both.

As stated above, the invention is based on the recognition that not the maximum of pore reduction but a critical pore volume concentration leads to optimum properties in the final articles. Therefore, the powder bodies compacted in conformity with this invention may well retain a comparatively large air or gas volume. The great advantage in using the same mold for the entire production sequence is therefore apparent, as less thoroughly compacted powder bodies do not possess that degree of mechanical resistance which is required for their safe transport from one mold into another mold.

The relationship between critical pore volume concentration and the hereby obtained properties has been determined by the inventor in connection with certain typical powder metallurgical materials according to the following tables. For these purposes these materials have been rampacked in an alumina container into barshaped specimens of different pore volume concentration, hereupon sintered and infiltrated in the same container. The transverse rupture strength in p. s. i. at room temperature and at a higher temperature and the deflection in inches caused by the maximum bend load at the higher temperature has been determined.

The result of this experimental work is given in the following tables.

TABLE I Properties of tungsten powder, rampacked to different pore volume concentration in alumina container, then sintered and infiltrated in same container with Nichrome alloy The data recited in Table I have been obtained by rampacking, sintering and infiltrating tungsten powder test pieces at various pore volume concentrations.

Within a pore volume concentration of between 4759% a sudden typical increase and an optimum of the transverse rupture and deflection has been observed; the critical pore volume concentration of tungsten therefore lies between about 47 and 59%.

TABLE H Properties of titanium carbide-nickel powder mixture, rampacked to difierent pore volume concentration in alumina container, then siutered and infiltrated in some container with Nichrome alloy Transverse Rupture, Pore Volume Concentration, fif

Percent 1 000 .9 6

Gold 1,000 0.

93, 500 62, 000 0. 089 132, 300 77, 800 O. 221 124, 600 81, 490 O. 158 103, 000 73, 100 O. 053 92, 500 72, 700 0. 012

The data given in this table have been obtained from test pieces made of titanium carbide 10% nickel powder mixtures rampacked to different pore volume concentrations, siutered and infiltrated in the same alumina boat with an alloy known by the trademark Nichrome (which according to page 229 of the ASM Metals Handbook, 1936 edition, comprises about 59% to 62% nickel, 10% to 14% chromium, about 0.1% carbon, and the balance substantially iron). The critical pore volume concentration lies between about 37 and 49%.

TABLE III Properties of titanium carbide-cobalt powder mixture, rampacked to differ nt pore volume concentrations in alumina container, then sintered and infiltrated in same container with Vitallium alloy Transverse Rupture, Specific Pore Volume Deflection Weight Concentration, in inches, Change in Percent 1,000 C. mgJcrnfi/ Cold 1,000 C. hr.

The data apparent from this table resulted from test pieces made of powder titanium carbide 10% cobalt mixture rampacked to different pore volume concentrations in an alumina boat, siutered and infiltrated in the same boat with an alloy known by the trademark Vitallium (which according to page 395 of Metal Progress, September 1947, comprises about 25% to 28% chromium, about 5% to 6% molybdenum, about 2% nickel, about 0.2% to 0.3% carbon and the balance substantially cobalt). The critical pore volume concentration lies between about 38 and 48%. The specific weight change is the one that occurred when exposing infiltrated material to air at 1000 C.

TABLE IV Properties of tungsten-chromium alloy powder (75% W,

25% Cr) rampacked to different pore volume concentrations, siutered and infiltrated in the same boat with Hastelloy C alloy Transverse Bupture, D fl f Pore Volume Concentration e Ion Percent 1 3 623 3 Cold 1,000 0.

The data given in this Table IV have been derived from the determination of the critical pore volume concentration of compacted sintered tungsten chromium alloy powders, and infiltrated with an alloy known by the trademark Hastelloy C which comprises the essential elements molybdenum not exceeding about 20%, chromium not exceeding about 18%, tungsten not exceeding about 6%, iron not exceeding about 7% and nickel essentially the balance (page 148, Engineering Alloys, published by ASM, revised 1945); the materials were alloyed at a ratio of 75% tungsten and 25% chromium. The critical pore volume concentration lies between about 48 and 51 TABLE V Properties of tungsten-chromium alloy powders (85 W,

% Cr) rampacked to different pore volume concentrations, sintered and infiltrated in the same boat with Hastelloy C alloy The data given in Table V refer to tests made on behalf of the critical pore volume of concentration of compacted, sintered tungsten-chromium alloy powders consisting of 85% tungsten and 15% chromium and infiltrated with Hastelloy C alloy. The critical pore volume concentration lies at between about 42% and 46%.

it is apparent that in the manufacture of high temperature articles from titanium and tungsten metal, from titanium and tungsten alloys and from compounds of these metals, that is their carbides, the critical pore volume may range from about 37% to 59%. When the skeleton body is produced from titanium carbide with nickel as the skeleton binder metal, the critical pore volume ranges from about 37% to 49%. Also, when the skeleton body is produced from titanium carbide with cobalt as the skeleton binder metal, the critical pore volume ranges from about 38% to 48%.

These data confirm the necessity and the great advantage of maintaining the critical pore volume concentration and using the same mold for all operations.

It will be apparent to those skilled in the art that the novel principles disclosed herein in connection with specific exemplifications thereof will suggest various other modifications and applications of the same. It is accordingly desired that in construing the breadth of the appended claims they shall not be limited to the specific exemplifications of the invention described herein.

What I claim is:

1. A method for the manufacture of heat resistant metal articles, such as turbine blades, buckets, nozzles, vanes and other structural gas turbine parts, in which finely divided, high melting refractory material powder selected from the group consisting of tungsten, molybdenum, tantalum, columbium, titanium and zirconium, mixtures of these metals, their refractory carbides and combinations thereof intimately mixed with a skeleton binder metal powder is shaped by pressure in a mold to a porous body which is thereafter sintered at an elevated temperature into a strong porous skeleton and thereafter infiltrated with a lower melting metal to form a dense article, which comprises providing said refractory material with particle sizes substantially all less than microns, compacting said material of controlled particle size to a critical pore volume falling within the range of about 37% to 59%, sinten'ng said compacted body into a strong porous skeleton and thereafter infiltrating said porous skeleton body with a heat resistant metal of lower melting point than the skeleton selected from the group consisting of iron, nickel, cobalt and chromium, mixtures of these metals, and mixtures thereof with tungsten, molybdenum and tantalum, whereby a heat resistant article is obtained having markedly improved room temperature and high temperature strength, and improved resistance to fatigue and to creep at elevated temperatures.

2. The method according to claim 1, in which the 10 heat resistant infiltrant metal is a precipitation harden= able alloy.

3. The method according to claim 1, wherein the finely divided refractory material of particle size substantially less than 20 microns has a controlled particle size distribution of about 9% less than about 1 micron, about ranging between about 1 and 5 microns, about 8% ranging between about 6 and 10 microns, and about 3% ranging between about 11 and 20 microns.

4. The method according to claim 1, wherein the finely divided refractory material, in which the particle sizes are substantially all less than 20 microns, has a controlled particle size distribution of about 55% less than about 1 micron, about 8% between about 6 and 10 microns, about 6% between about 11 and 20 microns and with not more than about 1% greater than 20 microns.

5. The method according to claim 1, wherein the skeleton body comprises substantially titanium carbide containing nickel as a skeleton binder metal.

6. The method according to claim 5, wherein the skeleton body comprises about titanium carbide and about 10% nickel as the binder metal, wherein the critical pore volume of the titanium carbide skeleton lies between about 37% to 49% of the volume of the body and wherein the infiltrant alloy comprises a nickel-base alloy containing about 59% to 62% nickel, about 10% to 14% chromium, about 0.1% carbon, and the balance substantially iron.

7. The method according to claim 1, wherein the skeleton body comprises substantially titanium carbide containing cobalt as a skeleton binder metal.

8. The method according to claim 7, wherein the skeleton body comprises about 90% titanium carbide and about 10% cobalt as the binder metal, wherein the pore volume of the titanium carbide skeleton lies between about 38% to 48% and wherein the infiltrant metal comprises a cobalt-base alloy containing about 25% to 28% chromium, about 5% to 6% molybdenum, about 2% nickel, about 0.2% to 0.3% carbon and the balance substantially cobalt.

9. The method according to claim 1, wherein the skeleton body comprises substantially tungsten metal and wherein the pore volume of the skeleton body lies between about 47% to 59%.

10. The method according to claim 1, wherein the skeleton body comprises a tungsten-rich tungsten-chromium alloy and wherein the pore volume of the skeleton lies between about 42% to 51%.

11. The method according to claim 10, wherein the tungsten-rich skeleton body contains about 5% to 35% chromium.

References Cited in the file of this patent UNITED STATES PATENTS 2,154,288 Scholz Apr. 11, 1939 2,193,413 Wright Mar. 12, 1940 2,313,227 De Bats Mar. 9, 1943 2,331,909 Hensel Oct. 19, 1943 2,363,337 Kelly Nov. 21, 1944 2,368,458 Engle Jan. 30, 1945 2,372,605 Ross Mar. 27, 1945 2,377,882 Hensel June 12, 1945 2,401,483 Hensel June 4, 1946 2,422,439 Scharzkopf June 17, 1947 2,435,227 Lester Feb. 3, 1948 2,456,779 Guetzcl Dec. 21, 1948 2,612,443 Goetzel Sept. 30, 1952 FOREIGN PATENTS 524,607 France Sept. 7, 1921 OTHER REFERENCES Wulf: Powder Metallurgy, published by American Society for Metals, Cleveland, Ohio, 1942, page 255 

1. A METHOD FOR THE MANUFACTURE OF HEAT RESISTANT METAL ARTICLES, SUCH AS TURBINE BLADES, BUCKETS, NOZZLES, VANES AND OTHER STRUCTURAL GAS TURBINE PARTS, IN WHICH FINELY DIVIDED, HIGH MELTING REFRACTORY MATERIAL POWDER SELECTED FROM THE GROUP CONSISTING OF TUNGSTEN, MOLYBDENUM, TANTALUM, COLUMBIUM, TITANIUM AND ZIRCONIUM, MIXTURES OF THESE METALS, THEIR REFRACTORY CARBIDES AND COMBINATIONS THEREOF INTIMATELY MIXED WITH A SKELETON BINDER METAL POWDER IS SHAPED BY PRESSURE IN A MOLD TO A POROUS BODY WHICH IS THEREAFTER SINTERED AT AN ELEVATED TEMPERATURE INTO A STRONG POROUS SKELETON AND THEREAFTER INFILTRATED WITH A LOWER MELTING METAL TO FORM A DENSE ARTICLE, WHICH COMPRISES PROVIDING SAID REFRACTORY MATERIAL WITH PARTICLE SIZES SUBSTANTIALLY ALL LESS THAN 20 MICRONS, COMPACTING SAID MATERIAL OF CONTROLLED PARTICLE SIZE TO A CRITICAL PORE VOLUME FALLING WITHIN THE RANGE OF ABOUT 37% TO 59%, SINTERING SAID COMPACTED BODY INTO A STRONG POROUS SKELETON AND THEREAFTER INFILTRATING SAID POROUS SKELETON BODY WITH A HEAT RESISTANCE METAL OF LOWER MELTING POINT THAN THE SKELETON SELECTED FROM THE GROUP CONSISTING OF IRON, NICKEL, COBALT AND CHROMIUM, MIXTURES OF THESE METALS, AND MIXTURES THEREOF WITH TUNGSTEN, MOLYBDENUM AND TANTALUM, WHEREBY A HEAT RESISTANT ARTICLE IS OBTAINED HAVING MARKEDLY IMPROVED ROOM TEMPERATURE AND HIGH TEMPERATURE STRENGTH, AND IMPROVED RESISTANCE TO FATIGUE AND TO CREEP AT ELEVATED TEMPERATURES. 